Rigid and Semi-rigid diaphragm in ETABS - Slab with beams

[Mina Kromer]

Dear Sir,

I noticed when applying a semi-rigid diaphragm to a slab with beams floor I got axial load in beams and slab from the vertical loading. But this axial force disappeared when I applied rigid diaphragm.

We used to ignore axial load in slabs and beams from vertical load even in manual analysis and design.

I am asking, what is the right, to ignore it or design for it, in case I needed to apply the semi-rigid diaphragm type ?

Another question, where these axial forces in slab and beams move when applying the rigid diaphragm to the slab, do it transfer to another type of stresses, for example, does it make an increase in moments and shear or what ?

I appreciate your reply.

Thank you

 

[steelnz2003]

Hi,
There is a vedio in the etab watch and learn you can view about this topic else this what I know

Semi-Rigid Diaphragm:

For semi-rigid diaphragm failure depends on connection strength between the diaphragm and the vertical elements like columns and walls.
This flexibility may result in a pattern different from the intended design that may change axial loads in the columns and transfer them to the beams and slabs.
If these axial loads are not paid attention to the end results can be wrong

Because of this, when a semi-rigid diaphragm is utilized, the axial loads in the beams should be considered in case of non rigid.

While in Rigid the diaphragm or the slab carries all the axial load on to the lateral resisting element.

Hope this helps

 

[Mina Kromer]

@steelnz2003

Thank you eng. for the reply.
If you can also help please, I am asking about the axial force appearing from the vertical load (not the lateral load).

 

[steelnz2003]

Hi Maina,
It depends on the value you will have some axial loads on the beams due to vertical load but they should be a minimum value. say less than 0.1 of the vertical load

 

[Celt83]

Rigid diaphragms do not deform, without deformation there is no strain, without strain there is no stress and without stress there is no internal forces.

A meshed slab allows for internal deformations. For a column supported slab there are moments generated at the top and possibly bottom of each column, these moments in turn generate shear in the columns which to satisfy statics at the column/slab joint generates axial forces in the slab.

 

[Backcheckrage]

It sounds like you're getting axial loads in beams due to gravity design with semi rigid turned on?
If there is, it shouldn't be much axial load at all.
I think you need to post some screen shots?...or ideally the EDB file?

Semi rigidity of diaphragm is a feature for lateral analysis only. There should be no effect on you gravity design load cases.

 

[STpipe]

So if I understand the methodology, you have the slab modeled as shells, with line elements embedded in the shell with common nodes. Your intent was for the shells to distribute the gravity loads to the beams? How does the bending moment in the beam compare to your hand calculation?

Did you apply any offsets on either the shell and/or the beam? If so, then you maybe getting composite action between the beam and the shell, therefore you will see axial loads in your beams. In that instance, to get the true bending moment, you would have to integrate the combined cross-section - essentially get the sum of the forces acting on the beam elements and the shell over a certain effective width.